1. Field of the Invention
The present invention relates to a group III-V type nitride compound semiconductor light-emitting device, and particularly to a group III-V type nitride compound semiconductor light-emitting device with blue or green band-edge emission.
2. Description of the Background Art
Heretofore, AlN, GaN and InN and especially AlGaN and GaInN of ternary system have been considered as materials used for a group III-V type nitride compound semiconductor light-emitting device. In particular, Japanese Patent Laying-Open No. 6-177423 discloses that a p-type AlGaN layer and an n-type GaInN layer are used to implement a blue LED or green LED.
Furthermore, "InGaN-Based Multi-Quantum-Well-Structure Laser Diodes", S. Nakamura et al., Jap. J. Appl. Phys. vol. 35 (1996) pp. L74-L76 describes a semiconductor laser device in which an AlGaN/InGaN-based laser with Ga.sub.0.8 In.sub.0.2 N/Ga.sub.0.95 In.sub.0.05 N multi-quantum well structure with 26 periods of quantum wells as an active layer is employed to achieve pulse operation at room temperature.
For LED which employs the combination of p-type AlGaN and n-type GaInN described above, the emission caused by the impurity level in GaInN is used to obtain blue or green emission. However, the emission caused by impurity level cannot achieve lasing when, as is in laser, high level injection of carrier is used, since band-edge emission is dominant. Furthermore, increasing a bandgap difference disadvantageously increases lattice mismatch.
For a laser which employs GaInN band-edge emission, the band discontinuity between a well layer of a quantum well structure and a barrier layer cannot be a sufficiently large due to the level in lattice mismatch between the well layer and the barrier layer, and the number of wells is thus considerably increased to 26 to confine the carriers. However, increasing the number of wells increases the width of an undoped active layer, and this extraordinary increases the value of threshold voltage and also results in increased lattice distortion in the active layer. Thus, while it is preferable that lasing can be obtained with a smaller number of wells, this requires an increased band discontinuity between a well layer and a barrier layer and selection of materials with a small lattice mismatches.
For an AlGaN presently used, the value of Al composition is a maximum of approximately 20%, and for that value the bandgap is approximately 3.87 eV and the lattice constant is 3.17 .ANG.. For an InGaN presently used, the value of In composition is approximately 15%, and for that value the bandgap is approximately 3.05 eV and the lattice constant is 3.24 .ANG.. The lattice mismatch therebetween is thus 2.2%.
FIG. 7 shows the relation between the lattice constant of a-axis and bandgap of a GaN-based semiconductor which contains Al or In. As is apparent from FIG. 7, as the bandgap difference between AlGaN and InGaN is increased, the lattice mismatch is also increased. It is thus impossible for the combination of AlGaN and InGaN to achieve a lattice mismatch of 0% and a large bandgap. In other words, for conventional AlGaInN-based semiconductors to increase the band discontinuity value between AlGaN and InGaN, the compositions of Al and In are increased and thus much more remarkable lattice mismatch results. It has thus been difficult for AlGaN/InGaN-based semiconductors to achieve lasing in the range from blue to green.